The availability of a truely tissue-specific growth promoter would provide a unique opportunity to investigate a wide variety of basic phenomena, including the control mechanisms involved in normal and malignant tissue growth and the role of growth promoters in the response to various toxins and tumor promoters. We have previously reported that normal weanling and regenerating adult rat livers contain a substance(s) (HSS) that stimulates liver growth in vivo and in vitro. An increase in the rate of DNA synthesis and an actual increase in cell number are observed and this stimulation is specific for the liver. The active material has been purified approximately 4500-fold by a combination of heat, ethanol and ammonium sulfate precipitation and ion exchange and gel filtration chromatography. The active material exhibits a MW of 13-20,000 and produces its effect only after a significant lag period, during which new RNA and protein must be synthesized in order for the stimulation of DNA synthesis to occur. The objectives of the current project are to complete purification of HSS to homogeneity and examine its mode of action. Purification will utilize isoelectric focusing, lectin columns, cyto-affinity and gel electrophoresis techniques. HTC cells will serve as the primary screening assay due to their marked sensitivity to HSS. However, active fractions will be tested on normal hepatocytes in culture and in vivo. A purified material will allow development of an immune assay and identification of the tissue source of HSS, its presence in various growth states (normal and malignant) and its possible production in response to toxins and tumor promoters.